Across Flats Bolt Size and Spanner Size Chart.
Understanding the correct spanner size for a bolt is essential for any fastener project.
The table below provides a comprehensive spanner size chart that correlates various hex bolt and head sizes with their corresponding spanner or wrench sizes, focusing on the across flats measurements. We also include data for how across flats measurements relate to head size and thread sizes, allowing to you understand what size spanner you will need for what size screw.
Across Flat Spanner Sizes
The term "across flats" refers to the distance between two opposite flat sides of a bolt head or nut. This measurement is crucial because it determines the appropriate tool needed for tightening or loosening. In particular, the across flat dimensions of bolts are vital when working with hexagon bolt head sizes.
The thread size of a bolt plays a significant role in determining both the bolt head size and then the required spanner size. The thread size refers to the diameter and pitch of the bolt's threaded portion.
Generally, as the thread size increases, so does the bolt head size and the necessary spanner size. By comparing the thread size, across flats sizes and spanner sizes in our chart, you can ensure compatibility among the bolts, nuts and tools used in your project.
Whether you're a professional tradesperson or a DIY enthusiast, understanding how spanner size and head size relate to thread size will aid you in selecting the proper equipment for your tasks. Refer to the chart below to find the exact across flat measurements, thread sizes, and corresponding spanner sizes for a variety of metric and imperial hex bolts.
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Imperial Thread Size
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Imperial Across Flats & Imperial Spanner Size (fractional)
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Imperial Across Flats (decimal inch) |
Metric Across Flats & Metric Spanner Size (mm) | Metric Thread Size |
|---|---|---|---|---|
| - | - | - | 2.5mm | M1 |
| - | - | - | 3mm | M1.2 |
| - | - | - | 3mm | M1.4 |
| #0 | 5/32 | 0.1560 | - | - |
| - | - | - | 3.2mm | M1.6 |
| - | - | - | 3.5mm | M1.7 |
| #1 | 5/32 | 0.1560 | - | - |
| - | - | - | 4mm | M2 |
| #2 | 3/16 | 0.1880 | - | - |
| - | - | - | 5mm | M2.5 |
| #3 | 3/16 | 0.1880 | - | - |
| #4 | 3/16 | 0.1880 | - | - |
| - | - | - | 5.5mm | M3 |
| #5 | 5/16 | 0.3125 | - | - |
| - | - | - | 6mm | M3.5 |
| #6 | 5/16 | 0.2500 | - | - |
| #8 | 11/32 | 0.3440 | - | M4 |
| - | - | - | 7mm | M4 |
| #10 | 3/8 | 0.3750 | - | - |
| - | - | - | 8mm | M5 |
| #12 | 7/16 | 0.4380 | - | - |
| - | - | - | 10mm | M6 |
| 1/4" | 7/16 | 0.4380 | - | - |
| 5/16" | 1/2 | 0.5000 | - | - |
| - | - | - | 12mm | JIS M8 |
| - | - | 13mm | M8 | |
| 3/8" | 9/16 | 0.5625 | - | - |
| - | - | - | 15mm | M10 |
| - | - | - | 16mm | ISO M10 |
| 7/16" | 11/16" | 0.6880 | - | - |
| - | - | - | 18mm | ISO M12 |
| - | - | - | 19mm | M12 |
| 1/2" | 3/4" | 0.7500 | - | - |
| - | - | - | 21mm | ISO M14 |
| - | - | - | 22mm | M14 |
| 9/16" | 7/8" | 0.8750 | - | - |
| 5/8" | 15/16" | 0.9375 | - | - |
| - | - | - | 24mm | M16 |
| - | - | - | 27mm | M18 |
| 3/4" | 1.1/8" | 1.1250 | - | - |
| - | - | - | 30mm | M20 |
| - | - | - | 32mm | M22 |
| - | - | - | 34mm | ISO M22 |
| 7/8" | 1.5/16" | 1.3125 | - | - |
| - | - | - | 36mm | M24 |
| 1" | 1.1/2" | 1.5 | - | - |
Tip From Our Engineers
“When it comes to measuring across the flats on hexagon bolts, using a caliper is more precise and less effort than multiple trips back and forth testing different spanners.”
Patrick Faulkner
Lead Engineer
FAQs
Q: What size wrench do I need for a bolt?
A: For a hex bolt, the wrench you'll need is one that matches the bolt's across flats measurement. The distance between two opposite flat sides of the bolt head. Refer to our spanner size chart to find the correct across flat size for your specific hexagon bolt size.
Q: How to find bolt size from spanner size?
A: To find a bolt size from the spanner size, you'll need to refer to our bolt size and spanner size chart. Using your spanner size, you can find the across flat head measurement of your bolt, then from that measurement you can find the thread size of the bolt.